1. Field of the Invention
This invention relates to telemetering information from a remote location to a control station. More particularly, the invention relates to the monitoring at a local test station the position of switches located at the wellhead of a subsea petroleum well.
2. Description of the Prior Art
In the control and monitoring of processes or instrumentation, the position of several remote switches or contact closure must be monitored. For example, to a monitor the production of one type of subsea petroleum well, the position of four valves on the subsea wellhead structure must be monitored at an accessible test location, such as on the sea surface or subsea atmospheric control system. Usually there are two limit switches associated with each valve, one to indicate the fully opened position and another for the fully closed position. Monitoring the position of these eight limit switches normally would require nine conductors including one common conductor and one conductor for each switch. Cable connectors are then required to have the same number of contacts.
However, due to the subsea environment of high pressure and salt water and the requirement to install the connectors using divers, submersible vehicles or other means, it is desirable to minimize the number of conducors and connector contacts for reliability and connect installation reasons. In addition, due to connector or cable corrosion, the value of the circuit impedance can change with time or from one installation to the next.
Thus, it is desirable to utilize only a single pair of electrical conductors between the subsea location and the test location. Furthermore, the use of alternating current electrical signals for monitoring is highly desirable to minimize electrolysis and subsequent corrosion of the conductors or connectors. Also, for long term reliability reasons, it is not desirable to use a source of power at the wellhead structure. It is likewise desirable to minimize circuit complexity for reliability reasons.
One prior art system for monitoring the status of control valves and switches utilizes passive resonant circuitry such as that disclosed in U.S. Pat. Nos. 3,550,090 issued to Baker, Jr., et al., 4,027,286 issued to Marosko and 4,268,822 issued to Olsen. In these systems, frequency selective resonant circuits are placed near the individual switches. The resonant circuits are connected or disconnected or the resonant requencies are changed by the closure of the switches. Decoding circuitry responsive to the change in resonance of the switch circuits provides the status of the switches. The coding and decoding circuitry is very complex. In addition, in the U.S. Pat. 4,027,286 system, the bandwidth of the conductive couper limits the channel capacity so that the status of only six switches can be determined.
Another prior art telemetry system is disclosed in U.S. Pat. 4,136,327 issued to Flanders, et al., wherein a two conductor line serves a dual purpose of supplying power into the borehole and communicating sensor data back up to surface. In the Flanders, et al. system, phase modulation is used to transmit a binary encoded signal to the surface that is received by a digital computer. In this type of system, the remote system electronics is complex, which may degrade reliability in a subsea environment.
U.S. Pat. 4,178,579 issued to Gibbens, et al. teaches the remote sensing of flow valves by using variable detection and a constant current source. The resistance of the entire sensing circuit is measured in a reference mode by passing a constant current through the circuit. The constant current is reversed by means of diodes, so that the current passes through the circuit containing the variable resistor. The difference in resistance from the reference mode is measured to determine the magnitude of the parameter of interest.
U.S. Pat. 4,103,337 issued to Whiteside discloses a data transmission system wherein a digital processor generates a date request signal across a two wire transmission line to a remote sensor. Sensor interface circuitry includes an analog to pulse width converter and a decoder which generates a pulse width signal in response to the data signal indicative of the value of the analog signal generated by the sensor. A pulse width to digital converter converts the data signal to digital form which is terminated in response to the pulse width signal from the sensor interface. The signal stored in the pulse width to digital converter is transmitted to the digital processor to indicate the value of the parameter being sensed.